1. Field of the Invention
This invention relates to a radiation image acquiring method and apparatus. This invention particularly relates to a radiation image acquiring method and apparatus, wherein composition processing is performed on a plurality of read-out images, each of which has been obtained from an image read-out operation for reading out an identical radiation image having been stored on a stimulable phosphor sheet, and a single composite image is thereby acquired. This invention also relates to a computer program for causing a computer to execute the radiation image acquiring method.
2. Description of the Related Art
Radiation image recording and reproducing systems utilizing stimulable phosphors have heretofore been known. With the radiation image recording and reproducing systems utilizing the stimulable phosphors, radiation carrying image information of an object, such as a human body, is irradiated to a sheet containing a stimulable phosphor (hereinbelow referred to as the stimulable phosphor sheet), and a radiation image of the object is thus stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored on the stimulable phosphor sheet during the exposure of the stimulable phosphor sheet to the radiation. The light emitted by the stimulable phosphor sheet is photoelectrically detected, and an image signal representing the radiation image of the object is thereby acquired. A visible image is then reproduced from the image signal and utilized for a diagnosis of the object.
Also, there has been known a technique for acquiring a composite image. With the technique for acquiring a composite image, in cases where a radiation image having been stored on a stimulable phosphor sheet is to be read out, a beam diameter of the stimulating rays is set to be smaller than a size of each of pixels in the radiation image to be read out, and the identical radiation image having been stored on the stimulable phosphor sheet is scanned a plurality of times with the stimulating rays and is thus read out. A plurality of read-out images, which represent the identical radiation image having been stored on the stimulable phosphor sheet, are thereby obtained. The thus obtained read-out images are then combined with one another, and a composite image, which represents the radiation image having been stored on the stimulable phosphor sheet and which is free from noise components, is thereby acquired. The aforesaid technique for acquiring a composite image is described in, for example, U.S. Pat. No. 4,748,326.
A dynamic range of a radiation image having been stored on a stimulable phosphor sheet is at least 105. Specifically, the dynamic range of the light intensity of the light, which is emitted by the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays having a predetermined intensity, and which represents the radiation image having been stored on the stimulable phosphor sheet, is at least 105. However, in general, the dynamic range of a sensitivity of a photoelectric converter for detecting the light emitted by the stimulable phosphor sheet is approximately 104. As a result, apart of the emitted light, which part has intensities corresponding to an approximately 101 part of the dynamic range of 105, falls outside a read-out sensitivity range of the photoelectric converter and is not capable of being detected appropriately. Therefore, there is a strong demand for a technique, with which the light emitted by the stimulable phosphor sheet is capable of being detected with a wide dynamic range, and with which an image representing a radiation image recorded with a wide radiation dose range is thus capable of being obtained.